The conventional laser printer is equipped with a fixed laser beam emitting element of which emission is controlled on the basis of image information. A rotatable polygon mirror deflects the laser beam emitted from the laser beam emitting element so as to produce a desired image on the photosensitive element. In other words, the deflection of the laser beam depends both on the angle which its optical axis makes with the facet of the polygon mirror, and on a distance between the polygon mirror and the photosensitive element, enabling the two-dimensional image to be described on the photosensitive element.
However, the conventional laser printer is indispensably equipped with a polygon mirror including a mechanical moving system which requires a rather complicated rotation control mechanism, a drive assembly, a large motor, and so on in order to stabilize its rotation. Further, the laser beam deflected by this polygon mirror needs a relatively long distance to reach the photosensitive element in order to attain a sufficient deflection, so that the polygon mirror must be located at quite a long distance from the photosensitive element. For such reasons, the conventional laser printer is necessarily large and heavy, necessitates inconvenient precise adjustments of mechanical moving assemblies, and so on.